When it comes to orthogonal frequency-division multiplexing (OFDM) systems, IQ imbalance is considered as one of the major impairment sources inside terminal radios. More attention should be paid to IQ imbalance, when direct conversion receiver (DCR) is not applicable. This may occur, for example, in carrier aggregation (CA) (especially, intra-band non-contiguous CA).
CA is being considered for the 3rd Generation Partnership Project (3GPP). In particular, the Long Term Evolution (LTE)-Advanced project considers intra-band non-contiguous CA where multiple carriers are separately placed within a band. Generally, there are two prominent radio architectures for user equipment (UE) being used for this purpose: DCRs and double conversion receivers. These two radio architectures each have their own pros and cons. A double conversion receiver enables a cost-efficient and hardware-efficient implementation, since multiple carriers may share the RF mixing stage and (part of) the IF mixing stage. However, one drawback of a double conversion receiver is that it is more susceptible to IQ imbalance. In essence, double conversion receiver requires a more balanced radio design at the expense of a more cost-efficient and hardware-efficient implementation.
IQ imbalance comes from imbalanced RF/analog circuitry, which includes local oscillators (LOs), mixers, filters, ADCs, etc. RF/analog circuitry may be calibrated by controlling some circuit parameters or the IQ imbalance may be compensated by digital signal processing inside digital baseband.